1. Field of the Invention
The present invention relates to an anaesthetic vaporizer.
2. Description of the Prior Art
An anaesthetic is usually administered to patients who, for various reasons, are about to undergo surgery or an examination. In inhalation anaesthesia, a gas mixture of oxygen and nitrous oxide are usually administered along with an anaesthetic. The anaesthetic is in liquid form and must be vaporized before it reaches the patient. Dosing and vaporization normally take place in an anaesthetic vaporizer.
A number of different types of vaporizers are known. One way to dose and vaporize liquid anaesthetic is to draw some of the gas mixture through the liquid, causing it to pick up anaesthetic vapor. Another way is to draw part of the gas mixture over the surface of the liquid anaesthetic. The surface area of this liquid can be enlarged with the aid of wicks which draws up the liquid.
Dosing in these vaporizer types is usually based on the saturation of gas, drawn over or through the liquid, with vaporized anaesthetic. Dosing is affected by factors such as temperature and pressure, in addition to problems in ensuring that saturation really does take place.
In another type of vaporizer, the liquid is first boiled or transformed into vapor in some other way. The vapor is then dosed via valves into the gas mixture supplied to the patient. Problems may occur in e.g. maintaining a constant pressure for the anaesthetic vapor (variations in pressure affect dosing).
A third version is to inject a dosed amount of liquid into the gas flow (directly or through a filter of some kind).
Dosing can be regulated by pressurization of the liquid or by pumping specific quantities of the liquid from a liquid container.
If a pump is used, it is essential for the pump to be exact and rugged. Pressure also plays an important role. The pump""s ability to pump the correct amount of liquid can be affected by pressure in the liquid container which, in turn, can be affected by the egress of liquid. Liquid held in the tubing between the liquid container and the pump also presents a problem.
An object of the invention is to achieve a rugged and reliable anaesthetic vaporizer that avoids the problems associated with known anaesthetic vaporizers, as described above.
The above object is achieved in accordance with the principles of the present invention in an anaesthetic vaporizer for dosing a liquid anaesthetic having a liquid container for the anaesthetic, an outlet tube proceeding from the liquid container to a dosing point, a first regulator for regulating the flow of anaesthetic from the liquid container toward the dosing point, a control unit for controlling dosing of the anaesthetic, a return tube connected in parallel across the first regulator at a first connection point downstream from the first regulator and at a second connection point upstream from the first regulator, a second regulator connected in the return tube, and a third regulator connected in the outlet tube, downstream from the first connection point. The control unit controls at least one of the first, second and third regulators for controlling dosing of the anaesthetic.
Connecting a return tube in parallel across the first regulator e.g. a pump, makes it easier to control pressure downstream from the pump. The second regulator is arranged in the return tube and the third regulator in the dosing tube. The control unit can dispense anaesthetic in a safe and accurate manner by controlling at least one of the regulators.
In an embodiment of the anaesthetic vaporizer according to the invention, the second regulator is an additional pump, and the third regulator is a dosing restriction. Pressure before the dosing restriction can be accurately regulated by regulating the pumps, flow through the dosing restriction thereby achieving great accuracy. Regulating one of the pumps is sufficient if the other pumps at a constant rate.
Alternatively, or as a complement, the dosing restriction can be variable. The control unit can then control the dosing restriction to compensate for any pressure variations occurring when there are rapid changes in dosing.
In another embodiment of the anaesthetic vaporizer according to the invention, the second regulator is a back-flow restriction and the third regulator is a dosing valve. The dosing valve can be controlled by the control unit according to the pressure, so the correct dosing is achieved. The pump can simultaneously be controlled to maintain the pressure.
As an alternative or a complement in this embodiment, the back-flow restriction can be variable.
The second connection point of the return tube can be arranged in the outlet tube upstream from the pump. The second connection point alternatively can be arranged at the liquid container.
The liquid container can be devised with a venting device in order to regulate the negative pressure in the container. It should be devised to minimize the risk of leakage of anaesthetic into the atmosphere.
Additional control of dosing (and an opportunity to increase accuracy) is achieved by arranging a flow meter in the outlet tube downstream from the first connection point.
A pressure stabilizer can be connected to the outlet tube, downstream from the first connection point, in order to enhance the maintenance of pressure downstream from the pump.
Arranging an anaesthetic meter to measure the anaesthetic content of the gas mixture increases safety and improves opportunities for refining dosing control.
Safety can also be increased by arranging a shut-off valve in the outlet tube downstream from the second regulator.